Buffering considerations

The size of the Dolby AC-4 audio main buffer in the receiver is assumed to be 120 KB per instance of the Dolby AC-4 decoder. (That is, for simultaneous decoding of two Dolby AC-4 bitstreams, each Dolby AC-4 decoder must have a main buffer 120 KB in size). The Dolby AC-4 bitstream encoding must be constrained so that a buffer of this size is always sufficient.

The 120 KB buffer size originates from the idea that Dolby AC-4 needs to buffer at most six frames until it can start decoding (according to the buffer signaling model). Some jitter is always possible, so one extra frame is added for safety, which then totals seven. The required buffer size is dependent on the bit rate and the frame rate. Therefore, the minimum buffer size can be calculated with the equation:

N = 7 × Ravg / Fframe

In this equation, Ravg and Fframe are the total bit rate and the frame rate of a Dolby AC-4 stream. For example, the data rate is 1,536 kbps and the frame rate is 23.976 fps, which makes the minimum buffer size 56.056 KB.

library_booksNote: The specified buffer size takes additional overhead into account; this is not reflected in the equation.